You will find that the appearance of the blade changes over time. Don't worry, the patina that forms is a normal reaction between the carbon steel and the air, as well as the properties of the food you cut. Your knife’s unique patina gives it a little character and some protection.
Make an effort to wash your knife straight after using it (by hand - not in the dishwasher!) and dry it immediately. Don't leave it on the draining board overnight. This risks the knife starting to rust. Your knife will not be damaged by rust spots as long as you gently scrub them off.
A knife is a cook's single most important tool. It should be treated with care. Carbon steel won't let you get lazy, because it insists on being treated right. It's a self-respecting metal like that. In return, it gives you one of the sharpest, hardest edges you could ever hope for.
Maintenance of your knives
1. Keeping your blade dry
Wipe your blade after every use with a dry cloth. The more you use your knife, the more it will create its own patina and protection.
2. Sharpening your blade
It is best to use a Japanese whetstone and a leather strop. This requires practice but it will give a much nicer edge and polish. Just follow the bevel I have already made to the blade.
3. Protecting the wood
Use linseed oil to buff your knife’s handle from time to time. For the blade, use camellia oil — like the Japanese samurai of old used to do. To store your knife, use your saya, a magnetic bar or a dry cloth for protection.
Sharpening your knife on a regular basis will keep the edge in great condition.
You can't use a regular steel honing rod on Japanese style knives. The steel I use is typically harder than the steel used in a cheap sharpening rod so it simply won't do anything, and you risk damaging your knife.
To get the ultimate edge on your knives, you are going to have to invest a little time in maintenance. Using a set of whetstones will get the best result.
This is an enjoyable part of owning and working with Japanese knives, and the satisfaction from a job well done is worth the effort. It's not as hard as you might think!